1. Technical Field
The present invention relates to a rheological fluid which is responsive to a magnetic field.
2. Background Art
Rheological fluids which are responsive to a magnetic field are known. Rheological fluids responsive to an electric field are also known. Such fluids are used in clutches, shock absorbers, and other devices. A characteristic of these rheological fluids is that, when they are exposed to the appropriate energy field, solid particles in the fluid move into alignment and the ability of the fluid to flow is substantially decreased.
Electric field responsive fluids and magnetic field responsive fluids include a vehicle, for instance a dielectric medium, such as mineral oil or silicone oil, and solid particles. Examples, of solid magnetic particles which have been heretofore proposed for use in a magnetic field responsive fluid are magnetite and carbonyl iron. The fluid also may contain a surfactant to keep the solid particles in suspension in the vehicle.
Silica gel is a form of silica which is very porous and thus has a large surface area. Silica gel is frequently used in electroviscous fluids which are responsive to an electric field, as the solid which is field-responsive.
U.S. Pat. No. 3,385,793 discloses an electroviscous fluid which is conductive. The fluid includes 30%-55% silica gel and 25%-35% silicone oil which functions as a vehicle. The fluid can also contain 1%-40% iron particles disclosed to function as a conductive agent. The composition is not described as one responsive to an electromagnetic field.
Other U.S. patents disclosing the use of silica gels in electroviscous fluids are U.S. Pat. Nos. 3,047,507; 3,221,849; 3,250,726; 4,645,614; and 4,668,417.
U.S. Pat. No. 2,661,825 disclose both ferromagnetic fluids which are responsive to an electromagnetic field, and which contain carbonyl iron; and electroviscous fluids which are responsive to an electric field and which contain silica gel. In the electroviscous fluids, the silica gel is used as the field-responsive solid, not as a dispersant. The electroviscous fluids comprise dry ground silica gel, a surfacant, such as sorbitol sesquioleate, a vehicle such as kerosene, and other ingredients.
U.S. Pat. No. 2,661,596 discloses a composition which is responsive to both electric and magnetic fields. The composition comprises micronized powders of ferrites, which are mixed oxides of various metals. The composition also contains dispersants and thixotropic agents. The patent also discloses the use of silica gel powder in an electric field-responsive fluid, and the use of iron carbonyl in a magnetic field-responsive fluid. There is no suggestion of the use of silica gel in a magnetic field-responsive fluid.
Other patents containing disclosures similar to that of U.S. Pat. No. 2,661,596 are U.S. Pat. Nos. 2,663,809 and 2,886,151.
A brochure published by GAF Corporation of Wayne, N.J., containing the code lM-785, captioned "Carbonyl Iron Powders", contains a discussion of carbonyl iron powders marketed by GAF Corporation. The iron particles are classified as "straight powders", "alloys", "reduced powders", and "insulated reduced powders". An example of a "straight powder" which is listed is an iron powder known as carbonyl "E".
A brief discussion is contained in the brochure concerning magnetic field responsive fluids. It is stated: "The spherically shaped particles of carbonyl iron presumably act like ball bearings in magnetic fluid coupling applications. The smallness of the iron particles gives larger surface area and more contacts than other powders and, hence, better transmission when locked. A lubricant and dispersant are generally required for best results." The brochure contains no disclosure concerning a preferred type of carbonyl iron or dispersant to be employed in a magnetic field responsive fluid.
A publication entitled "Some Properties of Magnetic Fluids", J. D. Coolidge, Jr. and R. W. Halberg, AIEE Transactions, Paper 55-170 (Feb. 1955), pages 149-152, discloses the use of different carbonyl irons in a fluid responsive to a magnetic field. The carbonyl irons disclosed include carbonyl "E" and carbonyl "SF", so-called straight powders, and carbonyl "L", carbonyl "HP", and carbonyl "C", all reduced powders. The article contains no disclosure concerning suitable dispersants, nor conclusions concerning the preference of one carbonyl iron over another in a magnetic field responsive fluid.
A publication entitled "The Magnetic Fluid Clutch" by Jacob Rabinow, NBS Tech. Rep. No. 1213 (1948) [also, Trans. Amer. Inst. Elec. Eng. Preprint 48-238 (1948)]discloses the use of hydrogen reduced iron and carbonyl iron "SF", a "straight" powder as indicated above. The publication contains no disclosure concerning suitable dispersants.
A publication entitled "The Magnetic Fluid Clutch" by S. F. Blunden, The Engineer, 191, 244 (1951) discloses the use of two grades of carbonyl iron, grade "ME" and grade "MC". Grade "ME" is said to be mechanically "hard" and grade "MC" is said to be mechanically "soft". No preference is given for one carbonyl iron over another.
A publication entitled "Further Development of the NBS Magnetic Fluid Clutch", NBS Tech. News Bull., 34, 168 (1950) discloses the use of carbonyl "E" powder in a magnetic fluid. Other compositional information concerning the fluid is also given.
Co-pending application Serial No. 372,293, filed June 27, 1989, assigned to the assignee of the present application, discloses a fluid composition responsive to a magnetic field which comprises a vehicle, and solid magnetic particles suspended in said vehicle. The fluid composition also contains a dispersant. A preferred magnetic particle is insulated, reduced carbonyl iron. A preferred dispersant is a fibrous carbon particle comprising intertwined carbon fibers having a length-to-diameter ratio in the range of about 10:1 to about 1,000:1. Preferably, the fibers have a surface area of about 300 square meters per gram.